Shrink wrap packaging machines, in general, are well known. There are two primary types of such machines. In a first type, a large roll of flexible plastic is rotatably mounted on a spindle at the first stage of the machine. The plastic material is folded in half along its longitudinal extent and the beginning of the roll is fed to a metallic forming head that spreads or opens the folded plastic so that the trailing edges thereof are spaced apart a distance sufficient to receive a moving package therebetween while the leading edge thereof presents a yieldable wall to the package. A conveyor belt means conveys the lead package into the wall formed by the leading edge of the plastic and the package travels forwardly, forcing the fixed position roll of plastic to rotate about its spindles which results in additional lengths of plastic being unrolled from the roll. The additional length of plastic surrounds the sides of packages following the lead package.
After the lead package passes a predetermined point, a sealing and cutting means seals and cuts the plastic behind the lead package, which seal and cut is forwardly of the second package. After the plastic has been sealed and cut forwardly and rearwardly of each package, the packages enter a high temperature tunnel where the plastic shrinks into a tight wrapping engagement around the packages to produce the final product. In this type of machine, the package is completely covered by the wrapping material.
In a second type of machine, no forming head is used and the plastic film is not folded. Instead, two rolls of plastic are used; a first roll is positioned above the conveyor means that carries the packages and a second roll is positioned below the plane of the conveyor means. The leading edges of each roll of plastic film are joined, and packages are then fed through the machine so that the first package impinges against the yieldable wall and starts the rotation of the rolls to commence the wrapping process.
In the second type of machine, the top, bottom, front and back of the packages are completely covered by the plastic film, but the left and right sides thereof are only substantially covered, there being a small, generally circular opening known as a "bull's-eye" in the wrapping on said sides when the packages exit the heat tunnel.
The wrapping industry uses more machines of the second type than of the first; the subject invention has utility in connection with both types of machines.
Conventional shrink wrap packaging machines of both types employ a single sealing and cutting element to form the seal and to cut the plastic before and after each package in a series of packages.
Typically, such machines operate at a speed that wraps in plastic about 60 packages per minute. However, the machines may run faster, up to 100 packages per minute, when smaller packages are being wrapped. Thus, the sealing and cutting elements (hereinafter referred to as the "sealing elements") reciprocate up and down at least 60 times per minute, and the conveyor belts run at an appropriate speed.
It is possible to speed up the rate of operation by simply running the conveyor belts at a higher rate of speed, coupled with a corresponding increase in the speed of operation of the sealing element. However, since the sealing element has considerable mass and has a reciprocating movement, the machine's rate of operation cannot be increased, as a practical matter, substantially beyond the machine's normal rate. Indeed, when conventional packaging machines of either type are run at an increased speed, they soon break down due to the mechanical stresses involved in high speed operation.
There are some multiple sealing and cutting station machines that have been built, but they do not follow the inventive methods disclosed herein. For example, there exists a multiple sealing station machine that employs a pair of sealing elements at longitudinally spaced sealing stations. Each element is mounted for rotary motion; to visualize the operation of rotary sealing element machines, one can envision a blade carried by a clock's second hand. Whenever an upper blade is in the six o'clock position and the lower blade is in the twelve o'clock position, the two blades will meet and form a seal and cut the plastic film. One obvious drawback of such machines is that the packages must be spaced rather far apart on the conveyor means due to the space required for the sweeping-motion blades.
Another multiple sealing and cutting station machine uses a square type of stroke like that of the present invention, but it also does not follow the inventive methods disclosed herein and as a result, it has numerous limitations and has not met with commercial success in the marketplace. As an example of its limitations, the space between packages on the conveyor belt means (which space is known as the "pitch") cannot be changed. In packaging machines, the ability to change the pitch so that the machine can accommodate differing package sizes is of primary importance. Moreover, the logic means required to maintain optimal conveyor belt speeds is complex in machines of this type. Due to these and other limitations, these machines have utility primarily in connection with smaller packages.
If the efficiency of both primary types (single and double roll) shrink wrap packaging machines could be doubled, by a means other than simply running them at twice the design speed, a new era of economical packaging could begin.
If such doubling of efficiency could be accomplished while still operating the sealing element at the relatively stress free, standard rate of 60 strokes per minute, (or whatever the machine's normal rate is) such an accomplishment would be a pioneering contribution to the art of packaging machines and methods.
However, the prior art is silent concerning how such a doubling of efficiency could be obtained.